Optical assemblies are often installed in a larger unit, such as for example an EUV illumination system. The optical element is in this case a three-dimensional geometrical body that is generally delimited by multiple surface areas and is used for performing one or more optical functions, such as for example the deflection or other manipulation of electromagnetic radiation. According to the optical beam path, the electromagnetic radiation is incident at least on one surface of the optical element. This surface is referred to hereinafter as the optically active surface. The optically active surface may be divided into multiple sub-surfaces, may be convexly or concavely curved in sub-portions and also include abrupt changes in topography, as are usual for example in the case of a diffractive optical structure. The optically active surface may include, among other things, a multilayer layer and/or other coatings, such as for example also an absorber layer. The multilayer layer may be formed via a contiguous coating vapour-deposited onto the optical element. The optically active surface may also be arranged on an MEM that is acting as an optical element. The individual optical elements may be constructed from different materials and/or generally also have different component geometries. During the operation of the illumination system, different illumination settings, such as for example an annular setting, a dipole, quadrupole or other multipole setting, can also be set by corresponding alignment of the optically active surface. This has the consequence that, depending on the illumination setting, electromagnetic radiation is incident on the optical elements with locally differing intensity. Furthermore, in the case of some of the optical elements considered here, locally differing absorber layers are applied. These absorber layers are used to achieve defined spatially resolved intensity distribution of the reflected radiation after the reflection of the EUV radiation at these optical elements. Depending on the degree of absorption of EUV radiation, the energy input at the optical elements may differ locally. During the operation of the illumination system, IR radiation from other optical elements or from mechanical components may also be incident on the optical element considered and be completely or partially absorbed there. The IR radiation may for example also originate from a heated component—such as for example a so-called sigma diaphragm—which has previously absorbed EUV radiation. Consequently, there may altogether be a locally differing heat distribution on the optically active surface of the optical elements. In particular, even with a homogeneous or symmetrical energy input, asymmetric local heat distributions may occur, if for example the absorption properties for electromagnetic radiation on the component differ in a spatially resolved manner. In the case where the optical elements are uncooled, sometimes temperatures of 200° C. and above can occur during the operation of the illumination systems. The spatially resolved heat distribution may result in undesired deformations of the optical elements or mounting bodies thereof. In particular when changing an illumination setting and when there is constant thermal loading over a certain period of time, deformations of the optical elements or the mounting bodies thereof may occur when considered over time in comparison with the basic state without thermal loading at ambient temperature, or some other previously set steady state or quasi steady state.